Minimal relaxation law for compaction of tapped granular matter

Collapsing granular beds: the role of interstitial air

A prefluidized sand bed consisting of fine particles compactifies when it is subjected to a shock. We observe that the response depends on both the shock strength and the ambient pressure, where, counterintuitively, at high ambient pressure the compaction is larger, which we connect to a decrease of the static friction inside the bed. We find that the interstitial air is trapped inside the bed during and long after compaction. We deduce this from measuring the pressure changes above and below the bed: The top pressure decreases abruptly, on the time scale of the compaction, whereas that below the bed slowly rises to a maximum. Subsequently, both gently relax to ambient values. We formulate a one-dimensional diffusion model that uses only the change in bed height and the ambient pressure as an input, and we show that it leads to a fully quantitative understanding of the measured pressure variations.

Microstructure evolution in density relaxation by tapping

The density relaxation phenomenon is modeled using both Monte Carlo and discrete element simulations to investigate the effects of regular taps applied to a vessel having a planar floor filled with monodisperse spheres. Results suggest the existence of a critical tap intensity which produces a maximum bulk solids fraction. We find that the mechanism responsible for the relaxation phenomenon is an evolving ordered packing structure propagating upwards from the plane floor.